In many geographical locations, it may be advantageous or at least desirable to utilize a conditioning system for a building, such as a home or the like for instance, which utilizes a heat pump having heating and cooling operating modes with a changeover device or means for respectively enabling the heating and cooling operating modes of the heat pump. Further and in conjunction with the heat pump, it may also be desirable to employ an auxiliary heating system, such as for instance an electrical heating apparatus or system, in order to provide comfortable heating of the conditioned space in the event of the occurrence of extreme or colder atmospheric temperatures. Of course, a thermostat which may be located in the conditioned space is necessary to control the operation of the conditioning system in the heating and cooling modes thereof, and in the past, various different types of thermostats were employed to control such conditioning systems of either the heat pump type or the heat pump and electrical heating apparatus combination. One such past thermostat is disclosed in the U.S. Pat. No. 4,114,681 issued Sept. 19, 1978 to Dann W. Denny which is incorporated herein by reference, and a dual thermostat adapted for operation to control the aforementioned conditioning system is disclosed in U.S. Pat. No. 4,115,751 issued Sept. 19, 1978 to Ronald G. Huizenga which is also incorporated herein by reference.
In some conditioning systems, it is advantageous or at least desirable to utilize both the electrical heating apparatus and a heat pump having first and second heating and cooling stages, respectively, with a changeover device to enable the heating and cooling stages or operating modes of the heat pump. Of course, the aforementioned two-stage heat pump is more efficient over a greater range of atmospheric temperatures so as to provide the desired comfortable heating and cooling operation of the conditioning system; however, in the event that colder atmospheric temperatures are encountered, the electrical heating apparatus may also be energized in conjunction with the heating mode operation of the two-stage heat pump. Thus, in the aforementioned conditioning system utilizing both the electrical heating apparatus and the two-stage heat pump with the changeover device therefor, it is necessary to employ a three-stage thermostat to control the heating and cooling operation modes of such conditioning system.